The present subject matter relates generally to vertical takeoff and landing (VTOL) technologies with quad independent and continuous-tilt rotors, and blended wing-body aircraft for use as an unmanned aerial vehicle (UAV).
UAV is a powerful aircraft that manipulates aerodynamic forces to provide lift without an onboard human operator. Generally, UAVs can be flown autonomously or piloted remotely, can be expendable or recoverable, and can be enabled to carry payloads.
VTOL is an aircraft that is subject to particular movement conditions including the abilities to vertically takeoff and land from a static position at ground level. VTOL aircrafts can hover in place and perform translational and rotational maneuvers while airborne. Additionally, VTOL aircraft can have the ability to transition between movement phases including vertical takeoff, hover, translational and rotational movement, and vertical landing. VTOL aircraft are advantageous because a smaller area is needed for takeoff and landing than conventional runway takeoff type aircraft. However, the transitions between movement phases of VTOL aircraft while airborne are known to create moment forces and other adverse aerial forces that cause disruptions to the stability of the VTOL aircraft.
Much research has been done to the UAV and VTOL technology to simplify the combined mechanical and control design to reduce the complexity and risk of failure while maintaining stability. It has been known to utilize multiple rotors to provide an improvement to vehicle balance and stability while in use. These vehicles must overcome stability issues relating to adverse moment forces caused by the environment as well as gyroscopic moment forces due to movement conditions and transitioning phases that are generated during aircraft maneuvers.
VTOL technology theoretically enables increased mobility and versatility compared to traditional fixed-wing aircraft. However, the most common VTOL designs, rotorcraft such as helicopters and quadcopters, generally have lower range and endurance compared to a fixed-wing aircraft due to active generation of lift. Existing VTOL designs which incorporate fixed-wing elements, and hence have better range and endurance compared to rotorcraft, tend to fall into 3 categories: quad-plane, tail-sitter, and tilt-rotor. However, such existing designs have instability problems and control difficulties, which decrease the versatility of the aircraft.
Quad-planes, also known as separate lift and thrust (SLT), consist of a fixed-wing design with forward propulsion unit(s), with separate vertical thrust units (usually 4) for VTOL capabilities. The quad-plane in flight has a transition zone where lift is transferred between active generation by the vertical thrusters and passive generation by the wings, as airspeed changes based on varying the thrust provided by the forward propulsion unit(s). During this transition zone, aerodynamic instabilities can occur as air separates and reattaches over the wing surfaces. The quad-plane typically has two control regimes: hover and forward flight, with the transition zone in between. In hover, the quad-plane stabilizes its position by varying thrust on the vertical units and varying the attitude of the aircraft to point the vertical units in various directions. However, due to the relatively large surface area of the wings, this design becomes unable to hold position in high winds, and in order to move forward, the aircraft must pitch down causing negative lift on the wings; or carefully vary the forward thrust while maintaining a positive pitch angle, however, this method does not react fast enough to stabilize the aircraft in gusting wind conditions. In order to move laterally, the aircraft must roll causing increased exposure of the wing area to crosswinds, which can cause the aircraft to become roll-unstable. Moreover, deceleration while maintaining a constant altitude is extremely difficult to control because the aircraft must pitch up so that the vertical units point backwards, and this increases the angle of attack of the wing, thereby increasing lift, followed by stall as the aircraft slows down. Finally, quad-planes control yaw rotation in hover by using differences in torque between the vertical thrusters, driven at different RPMs, which requires a large amount of thrust overhead as yawing torque is equal to rotor drag. During windy conditions, the amount of additional thrust required to maintain heading can easily overwhelm the available thrust and lead to instabilities.
The tail-sitter typically has one or multiple forward-facing propulsion units when the aircraft is in forward flight, and VTOL is achieved by pitching up to 90 degrees such that the same propulsion unit(s) are used to hover. As a result, the aircraft takes off and lands on its “tail” end. Similar to the quad-plane, such designs also have a transition zone between two control regimes. This introduces similar instabilities, difficulties in maintaining position in hover mode during high winds, and it is almost impossible to maintain altitude during the transition regions.
Conventional tilt-rotors use rotor thrust to control roll and pitch, along with optionally using differential tilt angle to control yaw, yielding greater controllability over other designs. However, most existing tilt-rotor systems transition between discrete hover and forward flight modes of operation, in which the rotors are tilted to fixed angles, and not coupled with attitude and position control. Discrete modes of operation prevent these systems from transitioning seamlessly between different airspeed regimes, and reduce translational controllability. Decoupled tilt and attitude control also allows the existing designs to be susceptible to the same stall and spin risks as conventional aircraft in the forward flight mode.
Accordingly, there is a need for a VTOL design that is capable of stable transition zones and avoids the disadvantages of traditional fixed-wing aircraft. There is a need to provide a VTOL UAV system that reduces undesired moment force phenomena generated by aircraft maneuvers to increase flight effectiveness, versatility, and efficiency while maintaining flight stability.